diff --git a/dlls/jscript/math.c b/dlls/jscript/math.c
index f7cf3d18807..99bd9f523a9 100644
--- a/dlls/jscript/math.c
+++ b/dlls/jscript/math.c
@@ -160,8 +160,22 @@ static HRESULT Math_abs(DispatchEx *dispex, LCID lcid, WORD flags, DISPPARAMS *d
 static HRESULT Math_acos(DispatchEx *dispex, LCID lcid, WORD flags, DISPPARAMS *dp,
         VARIANT *retv, jsexcept_t *ei, IServiceProvider *sp)
 {
-    FIXME("\n");
-    return E_NOTIMPL;
+    VARIANT v;
+    HRESULT hres;
+
+    TRACE("\n");
+
+    if(!arg_cnt(dp)) {
+        if(retv) num_set_nan(retv);
+        return S_OK;
+    }
+
+    hres = to_number(dispex->ctx, get_arg(dp, 0), ei, &v);
+    if(FAILED(hres))
+        return hres;
+
+    if(retv) num_set_val(retv, acos(num_val(&v)));
+    return S_OK;
 }
 
 static HRESULT Math_asin(DispatchEx *dispex, LCID lcid, WORD flags, DISPPARAMS *dp,
diff --git a/dlls/jscript/tests/api.js b/dlls/jscript/tests/api.js
index 71517cbb23e..a202fbdb9eb 100644
--- a/dlls/jscript/tests/api.js
+++ b/dlls/jscript/tests/api.js
@@ -626,6 +626,39 @@ tmp = Math.random(100);
 ok(typeof(tmp) == "number", "typeof(tmp) = " + typeof(tmp));
 ok(0 <= tmp && tmp <= 1, "Math.random(100) = " + tmp);
 
+tmp = Math.acos(0);
+ok(Math.floor(tmp*100) === 157, "Math.acos(0) = " + tmp);
+
+tmp = Math.acos(1);
+ok(Math.floor(tmp*100) === 0, "Math.acos(1) = " + tmp);
+
+tmp = Math.acos(-1);
+ok(Math.floor(tmp*100) === 314, "Math.acos(-1) = " + tmp);
+
+tmp = Math.acos(Math.PI/4, 2);
+ok(Math.floor(tmp*100) === 66, "Math.acos(Math.PI/4, 2) = " + tmp);
+
+tmp = Math.acos(true);
+ok(Math.floor(tmp*100) === 0, "Math.acos(true) = " + tmp);
+
+tmp = Math.acos(false);
+ok(Math.floor(tmp*100) === 157, "Math.acos(false) = " + tmp);
+
+tmp = Math.acos(1.1);
+ok(isNaN(tmp), "Math.acos(1.1) is not NaN");
+
+tmp = Math.acos();
+ok(isNaN(tmp), "Math.acos() is not NaN");
+
+tmp = Math.acos(NaN);
+ok(isNaN(tmp), "Math.acos(NaN) is not NaN");
+
+tmp = Math.acos(Infinity);
+ok(isNaN(tmp), "Math.acos(Infinity) is not NaN");
+
+tmp = Math.acos(-Infinity);
+ok(isNaN(tmp), "Math.acos(-Infinity) is not NaN");
+
 var func = function  (a) {
         var a = 1;
         if(a) return;